David Cameron speaks during a televised election debate in 2010 Getty Images

Britain’s broadcasters will “empty chair” David Cameron if he refuses to agree to new proposals for seven-way election debates.

In a robust statement the BBC, ITV, Sky and Channel 4, said they intended to screen three debates in the run up to polling day in May. The first two would include the leaders of the main political parties plus Ukip, the Greens, the SNP and Plaid Cymru. A third debate would be between David Cameron and Ed Miliband alone.

But significantly the statement goes on to say that if any of the invited leaders decide not to participate, “the debates would take place with those who accepted the invitation”.

The statement puts David Cameron in an invidious position. The Prime Minister and his aides have been desperate to avoid the debates going ahead and thought demanding the inclusion of the Greens could provide an insurmountable hurdle.

But the decision to include both the Greens and the nationalist parties makes it much harder for the Prime Minister to find a new excuse not to take part.

While the Democratic Unionist Party and other Northern Irish parties may contest their exclusion in the courts they are unlikely to be successful. None of the main UK parties field candidates in Northern Ireland so the broadcasters could argue that separate debates should be held in the province.

Proposed dates for the debates are April 2, 16 and 30 - with the final clash coming exactly a week ahead of the May 7 poll.

Labour have said they will accept the broadcasters recommendations while the Conservatives and the Liberal Democrats are still considering their responses.

TV executives previously suggested three debates: one head-to-head between Mr Cameron and Mr Miliband, another also involving Lib Dem leader Nick Clegg, and a third with Ukip's Nigel Farage thrown into the mix.

In a statement, the broadcasters said: "BBC, ITV, Sky and Channel 4 remain committed to holding election debates in the general election campaign. Following meetings with the Conservative, Labour, Liberal Democrat and Ukip representatives, it has not been possible to come to an agreement on the original proposal put forward by the broadcasters in October 2014.

"Since October, the broadcasters have together and individually had a number of meetings and conversations with the parties invited to take part, the Conservatives, Labour, Liberal Democrats and Ukip, and also discussions and correspondence with the SNP, Plaid Cymru and the Green Party. All these discussions have been constructive and useful in informing our thinking about the debates.

"Over the three months since the original proposal was put forward, the broadcasters have also continued to monitor the electoral landscape, as we promised to do, taking into account the polling evidence, and the expressions of public support for the debates to go ahead and for a wider range of parties to be included in the debates.

"In view of these factors, the broadcasters are now inviting party leaders to take part in the following debates within the official election campaign and approximately two weeks apart.

"Two debates between the leaders of the following parties: Conservative, Labour, Liberal Democrat, Ukip, Green, SNP and Plaid Cymru. One of these debates to be produced by ITV, and one by the BBC.